1. Field of the Invention
The present invention relates to an in-cylinder direct-injection spark-ignition internal combustion engine equipped with an open combustion chamber type piston in which fuel is injected directly into engine cylinders and it is ignited by an electric spark, and specifically to techniques for effectively sustaining strong tumble flow for stable combustion (stable stratified charge combustion and stable homogeneous combustion), and capable of enhancing the engine durability (e.g., a piston life).
2. Description of the Prior Art
Recently, there have been proposed and developed various in-cylinder direct-injection spark-ignition engines in which fuel is injected directly into the engine cylinders. Generally, on such direct-injection spark-ignition engines, a combustion mode is changeable between a homogeneous combustion mode (an early injection combustion mode) where fuel-injection early in the intake stroke produces a homogeneous air-fuel mixture, and a stratified charge combustion mode (a late injection combustion mode) where late fuel-injection delays the event until near the end of the compression stroke to produce a stratified air-fuel mixture. As is generally known, switching between these two combustion modes is determined depending on the engine operating condition, such as engine speed and load. Such stratified charging or stratified combustion mode is effective under a low engine-load operating condition where the amount of fuel injected is comparatively less. In contrast to the above, during high engine-load operation where the amount of fuel sprayed out is comparatively great due to demands for more engine power or more engine output torque, there is less requirement for stratified charging, and in lieu this it is necessary to form more uniform air-fuel mixture layers, particularly in order to avoid the engine from knocking. One such in-cylinder direct-injection spark-ignition engine has been disclosed in Japanese Patent Provisional Publication No. 8-35429 or in Japanese Patent Provisional Publication No. 6-81651. The Japanese Patent Provisional Publication No. 8-35429 teaches the use of a swirl control valve which creates swirl flow by modulating in-cylinder gas motion and modulates air/fuel mixture location. On the other hand, the Japanese Patent Provisional Publication No. 6-81651 teaches the use of upright straight intake ports which serve to efficiently direct intake air induced into the combustion chamber in a direction of a curved-top piston head, and to redirect the intake-air flow into a strong reverse tumble flow for stable combustion. In in-cylinder direct-injection spark-ignition engines disclosed in the Japanese Patent Provisional Publication Nos. 8-35429 and 6-81651, a deep piston-bowl-cavity combustion chamber is formed in a piston crown (or a piston head). Fuel-injection is made on the compression stroke, keeping the previously-noted in-cylinder swirl flow or the reverse tumble flow, and then the swirl flow (or the tumble flow) carries the air-fuel mixture to the vicinity of the spark plug. As a result, the rich mixture (a readily ignitable mixture) is concentrated around the plug and the concentration of the rich mixture is first ignited during the combustion stroke. Second, the surrounding air layers (lean or ultra-lean mixture layers of an air/fuel ratio close to a lean misfire limit) that contain little fuel, are ignited. To insure good stratified combustion (or to efficiently concentrate the readily ignitable mixture to the vicinity of the plug), it is not preferable to widely spread the fuel spray, injected from the fuel injector nozzle on the compression stroke, within the combustion chamber, for a particular period of time during which the fuel spray injected is carried to the vicinity of the plug. For the reasons set forth above, in conventional direct-injection spark-ignition engines, the piston-bowl-cavity combustion chamber is formed in the piston crown such that the center axis of the piston bowl cavity is largely offset from the central axis of the piston towards the intake valve, and such that the piston bowl cavity is formed as a relatively deep and large cavity. Additionally, in the prior art engines disclosed in the Japanese Patent Provisional Publication Nos. 8-35429 and 6-81651, a spray angle of fuel injected by the injector valve is narrow or less. Owing to such a less fuel-spray angle, the momentum of the sprayed fuel tends to be concentrated spatially, thus producing an undesiredly increased spray penetration (spray travel). Due to the excessively increased spray penetration, the fuel, which collides with or impinges on the piston head (i.e., the piston-bowl-cavity combustion chamber wall or the wall of the bowl-in-piston combustion chamber) and is reflected therefrom, may impinge secondarily on the cylinder wall during the homogeneous combustion mode in the intake stroke. Owing to such impingement of the narrow-angle fuel spray on the piston bowl cavity, there is an increased tendency for the incoming fuel to adhere to the piston-bowl-cavity combustion chamber wall in the form of a fuel film, and as a result rapid carbonization could occur, thus deteriorating exhaust-emission control performance by the increased amount of exhaust emissions such as smoke and particulate matter and by formation of unburned hydrocarbons (HC). There is a possibility that the increased spray penetration results in undesired deposits in the engine. The deep and large piston bowl cavity results in the increase in the overall surface area of the combustion chamber, and thus increases a thermal loss. Additionally, the off-center piston bowl cavity deteriorates a piston balance of the open combustion type piston. Particularly, during cold engine operation, there is a great difference between a thermal expansion efficiency of the engine cylinder and a thermal expansion efficiency of the piston, and thus the piston often experiences undesired wobbling motion. The wobbling motion of the piston, occurring during the cold engine operation, may produce noises and unsymmetrical piston wear.